In numerous electrical devices it is necessary to provide a liquid insulating medium which is called a "dielectric fluid." This liquid has a substantial higher breakdown strength than air and by displacing air from spaces between conductors in the electrical equipment or apparatus, materially raises the breakdown voltage of the electrical device. With the ever increasing sophistication of electrical equipment, the various electrical devices are operating at higher and higher voltages. This means that the dielectric fluids used in such devices are subjected to greater and greater stresses. These problems have, of course, necessitated the search for improved dielectric fluids.
By way of illustration, corona or partial discharge is a major factor causing deterioration and failure of capacitors or other power factor correction devices. A capacitor operating in corona will have a life of only minutes or hours instead of the expected 20 years. A capacitor properly impregnated with a suitable dielectric fluid will be essentially free of corona discharge to a voltage of at least twice the rated voltage. During use, when a dielectric fluid is placed under increasing stress a point is reached where breakdown occurs. The voltage at which the capacitor will suddenly flash into corona is known in the art as the corona inception voltage (CIV). This voltage is dependent upon the rate at which the voltage is applied. There is considerable difference between the sensitivity of different fluids to the rate of rise of voltage. The corona will, however, extinguish with a reduction of voltage. The corona extinction voltage (CEV) is not a fixed value for each fluid but is a function of the intensity of corona before the voltage is reduced. For best results both the CIV and CEV should be as high and as close together as possible.
With the exception of certain special applications, the polychlorinated biphenyl compounds (generally known as "PCB's") have been the standard dielectric fluid in electrical devices since the 1930's when the PCB's replaced mineral oil. Various other liquids including some siloxanes have also been suggested for use as dielectric fluids. See, for example, U.S. Pat. Nos. 2,377,689 and 3,838,056 and British Pat. Nos. 899,658 and 899,661. Recently the PCB's have lost favor in the sight of the environmentalists and efforts are being made world-wide to find suitable replacements for the PCB's.
Among the dielectric materials proposed as suitable replacements for PCB's are polyorganosiloxanes such as dimethylpolysiloxanes, methylphenylpolysiloxanes, phenoxy substituted methylphenylsilanes and siloxanes (e.g., U.S. Pat. No. 3,909,434), monochloroalkylsiloxanes (e.g., U.S. Pat. No. 3,838,056) and nitroarylsiloxanes (e.g., U.S. Pat. No. 3,900,416), employed either alone or in combination with various additive fluids such as soluble chlorendates (U.S. Pat. No. 3,948,789), ketones (U.S. Pat. No. 3,984,338) and the like. Unfortunately, these proposed "replacement" materials are frequently unacceptable in terms of one or more of the requisite high CEV and CIV values, and viscosity, flammability or fire point characteristics.
As one example, the electrical performance capability and high flash and fire points of 50 centistoke dimethylpolysiloxane fluid appear to make it well suited for use as a dielectric fluid in transformers. Such a silicone fluid is not readily usable in high stress (.gtoreq.1000 volts/mil) capacitors, however, because of its relatively low CEV of about 600 volts/mil. Put another way, once corona discharges are initiated in such a fluid, they will not extinguish because the operating stress substantially exceeds the CEV of the fluid and failure of the capacitor is certain to follow rapidly. As another example, use of volatile, low molecular weight organic additives may "fortify" the siloxane but substantially diminish desired flammability characteristics.
Specifically incorporated by reference herein is the disclosure of co-owned, co-pending U.S. application Ser. No. 836,448 filed Sept. 26, 1977, relating to novel capacitors including, as dielectric fluids, naphthoxy substituted siloxane fluids having a viscosity of less than about 50 centistokes at 25.degree. C. Also incorporated by reference herein is the disclosure of co-owned U.S. application Ser. No. 881,451 of applicant, filed concurrently herewith and relating to novel benzyloxy substituted silanes and siloxanes and electrical devices including the same as dielectric fluids.